1. Field of the Invention
The present invention relates to an apparatus and method for ink jet printing which executes printing by executing a scanning relative to a print medium using a print head, the print head having a nozzle array in which a plurality of ink ejecting nozzles are disposed.
2. Description of the Related Art
Various forms of printing apparatuses have been proposed or implemented which execute printing on print media such as paper or OHP sheets; these printing apparatuses are classified by a printing scheme for a print head. Print heads are based on a wire dot scheme, a thermal scheme, a thermal transfer scheme, or an ink jet scheme. Among these printing apparatuses, ink jet printing apparatuses have been gathering much attention; the ink jet printing apparatus uses a print head based on the ink jet scheme to jet ink directly onto print media, and thus requires reduced running costs and is very silent.
The ink jet printing apparatuses are roughly classified into a full line type and a serial type.
The full line type ink jet printing apparatus uses a long print head having a length larger than the maximum width of print media used. The full line type ink jet printing apparatus continuously conveys a print medium to form a predetermined image on the print medium. The full line type ink jet printing apparatus is thus suitable for high speed printing.
The serial type ink jet printing apparatus forms an image by repeating a main scan that moves a relatively short print head to form an image of a width corresponding to the length of the print head and a sub-scan that moves the print medium in a direction crossing a moving direction of the print head by a predetermined amount.
For these ink jet printing apparatuses, efforts have been made to further reduce the size of ink droplets ejected from nozzles and to increase the density of the nozzles, in order to allow the formation of high quality images of increased resolutions and reduced granular appearances. A print head has been developed which has a high density of 1,200 dpi and which ejects small droplets each of 4 pl. A printing operation with such a high density print head causes a landing position of droplets ejected from nozzles in the print head which are located close to its end, to be deviated toward the center of the print head (end deviation condition). The end deviation condition has not frequently occurred in printing apparatuses that eject larger droplets at a lower density.
With a print head of an increased density, the end deviation condition occurs both in the full line type ink jet printing apparatus and in the serial type ink jet printing apparatus.
In the manufacture of long print heads such as those used in the full line type ink jet printing apparatus, it is technically and economically difficult to densely arrange a large number of nozzles in a single substrate in a line. The full line type ink jet printing apparatus commonly uses what is called a long connecting head formed by connecting together a plurality of short chips having densely arranged relatively short nozzle arrays so that the chips are staggered.
However, in this connecting head, the end deviation occurs in each chip, making the density of a formed image uneven. In common connecting heads, nozzles are disposed so that the spacing between terminal nozzles in two adjacent chips is the same as that between two adjacent nozzles within the same chip (the latter is hereinafter also referred to as a nozzle pitch). In this case, the spacing between dots formed on a print medium by ink droplets ejected from the terminal nozzles in the adjacent chips is larger than that between dots formed by droplets ejected from two adjacent nozzles located close to a central part of the same chip. As a result, striped low-density portions (white stripes) are formed in the obtained image at intervals corresponding to the width of each chip. These white stripes degrade image quality.
Thus, a configuration has been proposed in which the chips are staggered and in which assuming the maximum deviation amount of ink droplets ejected from the ends of the chips, the ends of the adjacent chips are overlapped each other in the arranging direction. This configuration prevents possible white stripes even if end deviation occurs in droplets ejected from the ends of the adjacent chips because the ends of the adjacent chips are overlapped each other.
On the other hand, the serial type ink jet printing apparatus uses two printing schemes, one-pass printing and multipass printing. The one-pass printing is a scheme that completes an image in each scan area by one main scan of the print head. The one-pass printing is thus often used as a printing scheme that meets the recent demand for high-speed printing. However, images completed by the respective scans are sequentially joined together in the conveying direction of the print medium. Thus, with the one-pass printing, the end deviation condition results in uneven density portions (white stripes) at the connecting portions between images formed by the respective scans.
In contrast, the multipass printing completes an image on a same print area by executing a plurality of printing scans while changing which is used by the print head. The multipass printing can thus reduce possible density unevenness in the images. Further, a multipass printing scheme has been proposed which reduces the frequency with which the ejection nozzles at the end of the print head are used, while increasing the frequency with which the ejection nozzles in the central part of the head are used, to reduce the adverse effects of the end deviation condition, thus providing high quality images (see Japanese Patent Laid-Open No. 2002-96455).
Furthermore, to reduce density variations and density unevenness in the ink jet printing apparatus, following methods (1) and (2) have been proposed which stabilizes ejection speed and directionality (landing accuracy) as well as ejection amount per dot [pl/dot].
(1) Method for Controlling Ejection Amount
This is a method for divided pulse width modulation (method for PWM control) described in Japanese Patent Application No. 3-4713 proposed by the applicants. According to this method for divided pulse width modulation, a heat pulse that allows ink droplets to be ejected is composed of a pre-pulse that controls the temperature of the print head and a main pulse that allows ink droplets to be ejected. The pulse width of the pre-pulse is varied depending on the temperature of the print head. This makes it possible to inhibit a variation in ejection amount caused by a variation in temperature.
(2) Method for Correcting Density Unevenness
This method for correcting density unevenness uses the print head to print a test pattern at a fixed density and then reads the density unevenness of the test pattern. Then, on the basis of the read density unevenness, density signals for the nozzles are corrected. This is called a head shading method (HS method).
With the full-line type ink jet printing apparatus, having the long print head in which the ends of the adjacent chips overlap each other, it is possible to reduce possible white strips at the connecting portions between the chips. However, low-print-rate printing executed by each chip reduces the end deviation amount, possibly making the dot spacing smaller than the appropriate one, in contrast to high-print-rate printing. In this case, striped high-density portions (black stripes) having a printing density higher than that expressed by image data are printed in an image formed on the print medium. This degrades image quality. Further, the full line type ink jet printing apparatus completes an image onto the print medium by a single scan using the long print head. This prevents the division of one same scan area on the print medium into a plurality of portions for printing and a reduction in the frequency with which the ejection nozzles at the end of the print head are used, which are enabled by the serial type ink jet printing apparatus. It is thus difficult for the full line type ink jet printing apparatus to reduce density unevenness caused by the end deviation condition in the chips.
On the other hand, for the one-pass printing, the serial type ink jet printing apparatus also requires that the ends of the print areas printed by the print head overlap each other in order to avoid possible white stripes caused by a possible end deviation condition at the ends of the print ends. However, in this case, high-density portions (black stripes) having a printing density higher than that set by image data occur at the connecting portions between images formed by the respective scans. This degrades image quality.
Further, the technique disclosed in Japanese Patent Application No. 3-4713 controls the ejection amount of the print head to an average value to make it possible to eliminate a variation in density caused by a variation in temperature within a page or among pages. However, this technique cannot correct a variation in ejection amount among the nozzles of the print head. This prevents the elimination of the density unevenness within each nozzle array in the print head. In particular, the application of this technique to the serial type ink jet printing apparatus disadvantageously results in density unevenness at each connecting portion between images formed by the respective scans.
Moreover, the HS method in (2) prints a pattern of a fixed density (prints the pattern with the nozzles set at a predetermined print rate), then reads the printed pattern, and on the basis of the reading result, reads a correction value from a correction table for the fixed density. Then, on the basis of the read correction value, the density is corrected for the nozzles. This makes it possible to reduce the density unevenness near the fixed density. However, during an actual printing operation, the print rate of the nozzles varies every moment. Thus, the correction based on a pattern of a fixed density as described above does not enable the density unevenness to be sufficiently corrected. For example, a rapidly varying print duty or too high or low a print duty cannot be dealt with only by one correction table corresponding to a pattern formed at a fixed density. Consequently, the HS method requires a large number of correction tables that correct the density unevenness over the entire density area covering all densities from low density to high density. Providing these correction tables is difficult.
Thus, none of the conventional techniques sufficiently eliminate possible density unevenness on images. In particular, when pictorial color images or the like are printed on the basis of image signals (multivalue data) input by an external instrument via a read device or the like, density unevenness may occur. For example, if a full color image composed of four colors, cyan, magenta, yellow, and black, is printed by the serial type ink jet printing apparatus using a small number of passes, density unevenness may occur at the connecting portions between images printed by the respective scans. With the full line type ink jet printing apparatus, density unevenness may occur frequently at the connecting portions between images formed by the respective chips. If blue sky, sky at sunset, or human skin, which has a uniform tone, is printed, color balance is partly disrupted, changing the hue. The change in hue may result in color unevenness in images or degraded image color reproducibility (increased color difference). This degrades image quality. Density unevenness may also occur in monochromatic images in black, red, blue, green, or the like. Further, printing operations based on the multipass scheme is effective on image quality. However, this increases the number of scans executed by the print head, significantly reducing print speed.